1. Technical Field
This invention relates generally to methods and apparatus for high speed inspection of microelectronic device connections, and in particular to equipments used to illuminate interconnect wire bonds, ball bonds, and wedge bonds so as to improve discrimination against similar appearing backgrounds. This invention is related to application U.S. Ser. No. 914,541, now U.S. Pat. No. 5,302,836, which describes an earlier embodiment of this invention. That application included a multiple ring illumination system comprised of separate cylindrical fiber optics bundles which were angled and shutter switched to provide light at incident angles optimal for improved discrimination. See FIGS. 3-6 of U.S. Pat. No. 5,302,836. This invention further improves operational speed by replacing the fiber optics and shutter with a liquid crystal light valve assembly.
Typically, gold wires, gold bonds, and gold terminations are viewed against a gold background by manual inspection under a microscope. Since the field of view is normally less than one-eighth (1/8) of an inch, uniform illumination is needed to highlight the area of interest. The most common uniform illumination source is a circular ring light directing illumination perpendicularly or at a slight angle off normal onto the surface to be inspected. While this type of lighting works rather well under a microscope, the glares and shiny hot spots from the gold connections and the reflective backgrounds are often ignored by the operators, and they tend to interpolate, or "fill in" dark missing fragments of the images. Variations in such human judgments are a cause of inconsistent inspection results. For instance, the incremental fill in of a dark position on a wire may actually hide a break in the wire at that point.
With the recent development of machine vision technologies, attempts were made to inspect these gold interconnects and gold terminations per Mil-Std-883 Method 2017. Machine vision technologies, however, have not reached the sophistication of ignoring hot spots or filling in fragments in a random basis as a human being could. The approach, therefore is to develop an illumination technique, capable of isolating the specular interconnect wires, ball bonds, bond wedges and chips exclusively from its reflective backgrounds. In other words, provide a better contrast between the object of interest from its neighboring background. In addition, the illumination techniques must be fast enough to support machine vision technologies used for the image acquisition and processing of microelectronics inspection tasks.
2. Background Art
Early improvements in inspection methods were concerned with better work-piece illumination, more accurate location, object image acquisition, flaw identification, recognition and finally rejection against given criterion. The focus of evolutionary inventive steps in these directions may be seen by overview of the following patents:
The need for diffused lighting to reduce reflections and shadows in close-up photography has been long recognized. Shank, in U.S. Pat. No. 3,737,226 discloses apparatus in which an indirect light source illuminates a small object through a series of pyramidal reflectors. Light is thus diffused around four sides of the object before reflection to a camera. This invention was for close-up photography and would have limited value for high speed inspection of microelectronic elements.
Kanade et al., in U.S. Pat. No. 4,427,880, provide an array of discrete light-emitting sources which are used to sequentially illuminate a symmetrical work piece object. Reflections are focused on a light responsive position sensor so as to provide continuous indications of distance, surface orientation and curvature of the object. Details of surface geometry are not provided. This approach is most effective for measuring distance if the reflective surface is flat. Based on the position of the reflected light spots on the surface, the distance between the surface and the optical sensor can be calculated and determined. The system does not use continuous illumination for visual identification of the object and its orientation. U.S. Pat. No. 4,508,452 to DiMatteo et al provides for determining the surface profile of an object by projecting a pre-coded pattern onto the surface. By matching the newly acquired image pattern to a pre-determined image pattern, the profile of the newly acquired image can be extracted. An object surface is scanned by a moving projector and subdivided into the large number of coded sections. Comparisons are made of progressive photographs of the work-piece with those of a standard reference surface. The entire surface of an object may therefore be mapped. The system is not applicable to improving contrast between very small three dimensional objects, such as wires, and the reflective background.
Imamura et al. in U.S. Pat. No. 4,568,835, detects foreign matter such as dust particles on a plane substrate by means of scattering of the reflections from a laser beam. As a specimen work-piece such as a photomask is scanned by an oblique incidence laser illumination beam, reflections from foreign materials are less directly scattered than are those from the edges of the circuit pattern. The illumination incidence angle is 80 to 60 degrees off normal, with a portion of the beam being reflected from the substrate surface while the remainder is refracted into the substrate medium from which it is internally reflected then externally scattered outward. This approach does not consider circular illumination used with a highly reflective, low refractive background medium.
In a different surface measurement application, Schachar, in U.S. Pat. No. 4,695,163, determines the contour of a cornea by scanning the surface with coherent light from different positions along a rectilinear path. Reflections received by detectors along the track are maximally polarized when the incidence angle equals Brewsters's angle. From a knowledge of the index of refraction of the medium and of Brewster's angle, the relative spacial locations of points over the surface may be determined. The system should provide slow but precise information when a refracting medium is under inspection, but will have limited utility with highly reflective objects.
An object locating system for use with robotic systems is described in U.S. Pat. No. 4,791,482 to Barry et al. The system projects a known geometrical image from a light source onto the surface of an object. The plane of the image on the object is determined by finding a normal to the surface from known geometrical relationships. Comparison of normals at different surface points are used to calculate distances and angles between the points. Gaussian images are generated for comparison between referenced objects and the unit under test.
In the field of solder joint inspection systems, Sanderson in U.S. Pat. No. 4,876,455 discloses a fiber optic solder joint inspection approach, in which light from multiple sources is reflected from a specular object to a fixed array of transducers. The individual light sources are derived from a single source which is scanned and piped to a plurality of optical fibers which lead to individual openings spaced around a semicircular illumination frame. For a given surface attitude, reflections to the fixed transducers will result from only one illumination source, assuming essentially specular reflection from the surface. Given known surface features of the object, an approximate reconstruction of the shape is made. The point source is usable with solder joint fillet inspection, but not with the variably curved and positioned wiring connections of microelectronic assemblies.
A related invention, U.S. Pat. No. 4,988,202 to Nayar et al, extends the above approach to include generation of an Extended Gaussian Image representation of a solder joint which is then evaluated as to acceptability.
A system for inspection of the uniformity of the surface of a flat circuit board component such as a dual inline package, employing computer vision is taught by Chemaly in U.S. Pat. No. 4,972,493. Illumination is provided by low angular light at the surface edge. Anomalies on the flat surface of dual in-line packages are inspected for pits, holes, blisters, grease, marks, chips and cracks. Marks on the surface are distinguished from planned surface irregularities by comparison of grey scale brightness. The two directional lighting is not developed for specular surfaces such as wires, bonds and wedges.
Inspection of the circuit board components when soldered in place is taught by Ikegaya et al. in U.S. Pat. No. 5,027,418. Component lighting is provided by a standard ring illuminator positioned normal to the board. Board masking is provided to make an assessment of soldering condition independent of component lead placement on the circuit board lands.
It may be noted that none of the above inspection systems treat identification and inspection of variably curved and placed circuit elements such as microelectronic wires and bonds. Further, none teach the use or advantages of dual annular illumination sources, each disposed at different angular relationships with respect to normal, each of which provides optimal viewing contrasts for different classes of microelectronic wires and bonds relative their similar background.